The present invention relates to a process for controlling the boost pressure in an internal-combustion engine, supercharged by an exhaust-gas turbocharger of adjustable turbine geometry, to a predetermined desired value dependent on the operating point.
In a process known from Japanese Patent Document JP-A 60-56127, after a positive load change the adjustable turbine diffuser is transferred into a position reducing the flow cross-section, with the result that an improved response behavior of the exhaust-gas turbocharger is achieved. The disadvantage of this process is that the boost pressure p.sub.2, that is to say the pressure downstream of the compressor, which is a measure of the filling ratio and therefore of the engine torque, does not increase with the speed, as is true of the pressure p.sub.3 upstream of the turbine (exhaust-gas counterpressure). This leads to the occurrence of uncontrolled p.sub.3 rises. In these operating ranges, therefore, the internal-combustion engine has to push out against a relatively high exhaust-gas counterpressure, this entailing a reduction of efficiency.
To reduce the fuel consumption as a result of too high an exhaust-gas counterpressure level after the reduction of the flow cross-section, according to German Patent Document 3,624,248 the turbine diffuser is transferred continuously in the direction of the opening position again with the increase in the boost pressure due to the load increase, specifically until a boost pressure corresponding to the instantaneous load predetermination point has been set. Since the behavior of the exhaust-gas counterpressure p.sub.3 after the change of the load predetermination is ignored completely here, the undesirable p.sub.3 rises occur here too as a result of the relatively slow increase of the boost pressure p.sub.2 in comparison with the exhaust-gas counterpressure p.sub.3.
An object of the present invention is to provide a process for controlling the boost pressure by which, in the non-steady-state mode of the internal-combustion engine, especially after a positive load change out of low load and speed ranges, an improvement in the efficiency of the internal-combustion engine is obtained.
This and other objects are achieved by the present invention which provides a process for controlling the boost pressure in an internal-combustion engine, supercharged by an exhaust-gas turbocharger with adjustable turbine diffuser, to a predetermined desired boost-pressure value dependent on the operating point. After a positive change of the internal combustion engine load, the turbine diffuser is transferred into a position reducing a flow cross-section onto the turbine. After the positive load change during a non-steady-state mode, boost pressure is regulated according to a first characteristic, corresponding to the instantaneous true deviation of the actual boost-pressure value from the desired boost-pressure value, for the regulating variable for controlling the turbine diffuser. The regulation of the boost pressure takes place according to the first characteristic only below a predetermined threshold value for the exhaust-gas counterpressure upstream of the turbine. After the predetermined threshold value has been exceeded, the boost pressure is regulated according to a second characteristic, the second characteristic being based on a hypothetical deviation of the actual boost-pressure value from the desired boost-pressure value, the amount of which is larger than the instantaneous true deviation.
The process according to the present invention prevents the possibility that, after a positive load change, uncontrolled pressure rises will occur in the exhaust-gas line upstream of the turbine during the actual increase in the boost pressure (p.sub.2) Since the internal-combustion engine consequently no longer has to push out against a raised exhaust-gas counterpressure, an increased efficiency of the internal-combustion engine is afforded. Furthermore, with the process according to the present invention a simple and therefore cost-effective limit-pressure switch changing its switching state at a specific limit pressure p.sub.3,G can be used as a transmitter for the exhaust-gas counterpressure p.sub.3.
The objects of the invention are also achieved by the present invention which provides an arrangement for controlling the boost pressure in an internal-combustion engine, supercharged by an exhaust-gas turbocharger with adjustable turbine diffuser, to a predetermined desired boost-pressure value dependent on the operating point. The arrangement comprises a compressor arranged in an intake line, a turbine arranged in an exhaust gas line, and an electronic control unit which receives a first signal corresponding to a current internal-combustion engine load and a second signal corresponding to a current internal-combustion engine speed and which generates a regulating-value signal for controlling the turbine diffuser. A pressure sensor is arranged in the intake line downstream of the compressor and is connected to the electronic control unit via a measured-value line. A pressure switch is arranged in the exhaust-gas line upstream of the turbine and is coupled to the electronic control unit via a line. The pressure switch has two switch positions, a switching state of said pressure switch being transmitted to the electronic control unit via the line.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.